1. Field of the Invention
The present invention relates to a super-capacitor, and more particularly, to a super-capacitor electrode.
2. Description of Related Art
Super-capacitor is a special type of energy storage unit being developed in recent years, which is based on the electrochemical reaction at the electrode/solution interface, with characteristic between traditional electrostatic capacitor and chemical power supply.
In the manufacture of a super-capacitor, an electrode is generally prepared by applying a prepared slurry containing an active material onto a current collector. After cutting the electrode and riveting leads, electrodes are stacked with separator papers in the order of separator paper, electrode, separator paper, and electrode, and then wound to form a capacitor core; and then the capacitor core is placed into a shell, and the shell is assembled with an end-cover. Sequentially, after the above assembled capacitor core is filled and impregnated with an electrolyte through a filling hole, the filling hole is sealed. Thus, a super-capacitor is made-up.
In the manufacture of a super-capacitor, a key technique is the electrode material and the manufacturing process for the electrode. Generally, a mono-layer coating method is employed in the manufacturing process of the super-capacitor electrode, i.e., directly applying an active material slurry onto a current collector. Since the internal resistance of the capacitor is affected by the bonding tightness between the active material layer and the current collector, it is required for the monolayer coating method to add more adhesive agent into the active material layer. Thus, for the super-capacitor whose size specification is fixed, since more adhesive agent is required and thereby the amount of the active material is reduced, the energy density of the super-capacitor would be decreased.
In order to enhance the conductivity and the adhesive strength between the active material layer and the current collector, and at the same time to reduce the amount of the adhesive agent used in the active material layer, a method for manufacturing a super-capacitor electrode is disclosed in the Chinese Patent CN101562078A. In this method, an adhesively conductive layer is applied between an active material layer and a current collector. Thus, the conductivity and the adhesive strength between the active material layer and the current collector are enhanced; meanwhile the amount of the adhesive agent used in the active material layer is reduced.
In the patent mentioned above, it is disclosed that the current collector is made from an aluminum foil, a corroded aluminum foil, a copper foil, or a nickel foil. And in this patent, by comparison of the aluminum foil (also referred to as the uncorroded smooth aluminum foil) with the corroded aluminum foil as a current collector, it is found that the super-capacitor using the uncorroded smooth aluminum foil as a current collector has a higher internal resistance than the super-capacitor using the corroded aluminum foil. The reason is that the surface of the uncorroded smooth aluminum foil results in a weak adhesive strength for the active material applied on it. Thus, it is difficult to apply the active material on the surface of the uncorroded smooth metal foil, or, even if the active material layer is applied on the surface of the uncorroded smooth metal foil, the active material layer would peel off rapidly at the course of charge-discharge cycle of the super capacitor product, and thereby the life time of the product is shorten.
In consideration of enhancing the bonding tightness between the active material layer and the current collector, if the uncorroded smooth metal foil is kept unchanged, the amount of an adhesive agent or an adhesively conductive agent must be consequentially increased. So, the demerit still emerges as in the aforementioned monolayer coating method. That is, the increasing of the amount of the adhesive agent or the adhesively conductive agent will affect the amount of the active material used under the fixed size specification of the super-capacitor, and accordingly, it is required to perform a compromise between the amount of the conductive agent and that of the active material. Accordingly, the electrode prepared in this way will not attain perfect overall performance.
In consideration of enhancing the bonding tightness between the active material layer and the current collector, if the adhesive agent is kept unchanged, the uncorroded smooth metal foil should be changed. In the developing history of capacitor, from the initial aluminum electrolytic capacitor to the present super-capacitor, it is a traditional way to use a corroded metal foil as a current collector. One of the important reasons for the use of the corroded metal foil in the aluminum electrolytic capacitor is to increase the electrode area and the capacitance. The main purpose of using the corroded metal foil in the super-capacitor is to enhance the adhesive strength of the active material layer via porous surface of the corroded metal foil, but the increasing of the electrode area is realized by high specific surface area of the active material. However, the capacitor using the corroded metal foil has the disadvantages as follows: (1) due to low mechanical strength and poor processability, the corroded metal foil is apt to wrinkle and break-off; (2) due to a corrosion process of the corroded metal foil, the corroded metal foil is far expensive than the uncorroded metal foil; (3) for ensuring enough mechanical strength, the corroded metal foil is required to be thicker than the uncorroded metal foil, which results in the decrease of the energy density of the product.
Thus, it can be seen that, due to the difficulty caused by the above compromise between the current collector itself and the active material layer, the electrode manufactured by the conventional technique would not attain perfect overall performances.